Generally, among the writing devices which are used for image forming apparatuses exhibiting an electro-photographic process, a writing device is well known in which LED print heads (hereinafter referred, to “LPH”), having an LED array structured of a plurality of LEDs, are used, and which exposes an image onto a photoconductor to form a latent image. In said LPH, a converging-rod-lens array, generally called a “SELFOC lens”, is used to concentrate light rays so that high resolution is obtained. If the distance between the LPH and the photoconductor varies, the image exposed on the photoconductor becomes out-of-focus, and the resolution is reduced. Further, in typical color image forming apparatuses which superpose plural color images, color unevenness usually tends to occur.
In the above-described writing device, in order to obtain preferred type of images, positional accuracy within ±0.1 mm is generally essential for the position of the LPH with respect to the focal surface on the drum of the photoconductor. The more the PLH approaches the heat focus, the more the image quality improves. Therefore, the positional adjustment of the LPH requires precise control.
However, in the writing devices which has been conventionally used for the image forming apparatuses, such as copy machines and printers in business offices, the accuracy for assembling the LPH onto the photoconductor tends to be controlled only by the manufacturing accuracy of mechanical parts to support the LPH. In such cases, since the focal depth of the LPH is generally ±0.06 mm, the accuracy for manufacturing the member to support the LPH is not enough to realize the above targeted positional accuracy which is within ±0.1 mm.
Unexamined Japanese Patent Application Publication No. 58-205171 discloses an electrostatic printer having a print head unit, in which a light emitting section and a SELFOC lens are mated, and having a positional adjustment member which enables precise adjustment of the assembling position of said print head unit onto the electrostatic printer.
Further, Unexamined Japanese Patent Application Publication No. 62-242,929 discloses a writing device having a light emitting section on a frame, and the light emitting section is adjustable, being supported by a positional adjustment member.
In Unexamined Japanese Patent Application Publication No. 58-205171, when the print head unit is to be assembled onto a mounting member of the electrostatic printer, a positional adjustment screw of the mounting member is rotated so that the bottom face position of a mounting plate of the print head unit can be moved, whereby after the mounting position of the print head unit is adjusted, the print head unit is fastened by a fastening screw. Due to this structure, the distance between the print head unit and the photoconductor is controlled. However, since the positional adjustment screw and the mounting member are not firm contact with each other, they are not stable in relation to each other, that is, the movement of the print head unit becomes unstable, which results in inaccurate positioning of the print head unit. Further, when the above cited screw is fastened, the position of the print head unit in the longitudinal direction, (which is the axial direction of the photoconductor) may shift, which results in unstable repeatability of the adjustment, as well as an out-of-focus of the produced image.
In Unexamined Japanese Patent Application Publication No. 62-242929, a supporting member to support an led array head is allowed to come into contact with a cylindrical positional adjustment member, having an eccentric hole, mounted on a frame, and said positional adjustment member is then rotated. By this structure, the supporting member is moved so that the distance between the LED array head and the photoconductor is adjusted, and the supporting member is mechanically fastened onto the frame by a fastening screw and nut. However, since the supporting member and the positional adjustment member are not in firm contact with each other, they are not stable. The movement of the LED array head tends to become unstable, which results in inaccurate positioning. Further, when the fastening screw and nut are fastened, the supporting member tends to be declined so that the position of the LED array head is shifted in the longitudinal direction, which tends to result in an out-of focus of the produced image.